An automatic transmission (commonly "AT" or "Auto") is an automobile gearbox that can change gear ratios automatically as the vehicle moves, freeing the driver from having to shift gears manually. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment.

Most automatic transmissions have a set selection of possible gear ranges, often with a parking pawl feature that will lock the output shaft of the transmission. Continuously variable transmissions (CVTs) can change the ratios over a range rather than between set gear ratios. CVTs have been used for decades in two-wheeled scooters but have seen limited use in a few automobile models. Recently, however, CVT technology has gained greater acceptance among manufacturers and customers.

Some machines with limited speed ranges or fixed engine speeds, such as some forklift trucks and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.

Comparison with manual transmission

main: l1=Comparison of manual and automatic transmissions In most Asian markets, automatic transmissions have become very popular since the 1990s. Most cars sold in the United States since the 1950s have been equipped with an automatic transmission, though it remains common practice to advertise the cheaper pricing based on manual transmission and treating automatic as an upgrade. However, manual transmission is still more popular than automatic in Europe. Automatic transmissions are easier for drivers, especially beginners. In some jurisdictions, drivers passing their driving test in an automatic-transmission vehicle will not be licensed to drive a manual-transmission vehicle. Examples of this are in the United Kingdom, Lithuania, Ireland, Estonia, the Netherlands, Sweden, New Zealand, Norway, Poland, Australia, South Africa, Trinidad and Tobago, People's Republic of China, Republic of Korea, Singapore and Israel. However, a driver holding a manual license is permitted to drive an automatic.

Automatic transmission modes

Conventionally, in order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the floor next to him/her. In order to select gears/modes the driver must push a button in (called the shift lock button) or pull the handle (only on column mounted shifters) out. Some vehicles (like the Aston Martin DB9) position selector buttons for each mode on the cockpit instead, freeing up space on the central console. Vehicles conforming to US Government standards must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, quadrant-selected automatic transmissions often utilized a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to unintentional gear mis-selection, as well the danger of having a selector (when worn) jump into Reverse from Low gear during engine braking maneuvers.